During the off-season teams are allowed to carry up to 90 players through training camp and only the top 53 cap values will count against the cap (similar to the NFL's Rule of 51). Once the regular season begins, teams can have up to 55 active players but only the top 53 count against the cap (like NFL team activating 2 extra practice squad players on gameday).
A player's salary cap value = Current Season Annual Salary + Roster Bonus + (Signing Bonus / Yr)
(CAP VALUES from top 53 players)
+ 42,135,000 (EMPTY ROSTER SPOT CHARGES if under 53 active @ 795K during regular season)
+ 0 (INACTIVE - see roster page)
+ 0 (RETIRED - see roster page)
+ 0 (CAP HITS - see roster page)
= 42,135,000 (TOTAL SALARY CAP SPENDING: TOP 53 SALARIES + CAP HITS)